1. FIELD OF THE INVENTION
The invention relates generally to the technical field of silicon nitride thin films and their applications, and more particularly to a technique for industrial preparation of silicon nitride films with performances that are considerably improved relative to the prior art.
2. DESCRIPTION OF RELATED ART
Silicon nitride thin films are known. They find uses in electronics, in particular for the passivation of integrated circuits and as insulating films in thin film transistors (TFT), such as those of flat screens. The technique most generally used for their preparation is chemical vapor phase deposition at low pressure and with a plasma (CVD under plasma). The chemical types introduced in the plasma are, among others, nitrogen (N.sub.2) and silane (SiH.sub.4) optionally diluted in helium (He). The frequencies of the plasma-generating electric fields are most commonly 13.56 MHz.
The performances of the silicon nitride films which are desired relate to several criteria, which include their composition, their density, their resistance to etching and their role as a barrier against the diffusion of oxygen and alkalines. Their structure is often characterized by the index of refraction and it is desired that the index be as close as possible to the index of the solid Si.sub.3 N.sub.4.
The technical problem that the invention trys to solve is to provide a method for industrial production of silicon nitride films with improved performances, and in particular according to the the preceding criteria.
The silicon nitride films used in electronics play a role, not only during the operation of the finished device, for example, as an insulating film between two active films (grid insulating material of a TFT) but they also play a very important role during the production of the device itself. This role is that of a protective film. The production techniques of thin-film transistors, such as those which constitute the nodes that are found at each pixel of a flat screen are actually very violent, such as high-temperature heat treatments and etching with hydrofluoric acid. During these operations, silicon nitride thin films are expected to play the role of impassable barriers either to mark preferred zones that are not desired to be attacked or to avoid during heating, the degradation of the amorphous or polycrystalline silicon that they are supposed to protect. It would even be desirable for the silicon nitride films to act as a barrier against the diffusion of alkaline ions, which would allow their use as an encapsulating film for a glass substrate.
The techniques used to produce the silicon nitride thin films are usually those of CVD under plasma. Such a technique is described, for example, in the publication "Plasma-enhanced CVD of high quality insulating films" by J. BATEY et al. in Applied Surface Science 39 (1989), 1-15 North-Holland, Amsterdam. The deposition is performed from a gas which is a mixture of SiH.sub.4 diluted in helium (2%) and nitrogen in close proportions, this mixture itself being further diluted in helium (from 20 to 60 times). The gas circulates in an enclosure between two plane electrodes separated by several cm where the pressure is about 130 Pa, and subjected to an alternating electric field of 13.56 Mhz with a power density on the order of 20 mw/cm.sup.2. A substrate, with a monocrystalline silicon base, for example, optionally provided with other thin films, is heated up to temperatures of 350.degree. C. and silicon nitride is formed at its surface.
A method using the same technique with a gas mixture containing ammonia with a frequency of 50 Khz is also described in: "W. A. P. CLAASSEN, Ion Bombardment-induced Mechanical Stress in Plasma Enhanced Deposited Silicon Nitride and Silicon Oxynitride Films, Plasma Chemistry and Plasma Processing 7 (1987) March, no. 1, Bristol G. B."
A different method using the electron cyclotron resonance plasma CVD (ECR plasma CVD) is described by Y. MANABE and T. MITSUYU in "Journal of Applied Physics 66 (1989), Sep. 15, no. 6." This technique makes it possible to obtain silicon nitride films that are very high-performing but under specific laboratory conditions and which make the application limited to small dimensions.
To test the performances of silicon nitride thin films, two test methods are used in particular: hydrofluoric attack and SIMS analysis of the film after the diffusion test. The first method consists in measuring the thickness of the film as it evolves over time during a retention in a bath consisting of an aqueous solution at 35% by volume of NH.sub.4 F and 6% of HF. The bath is thermostated at 28.degree. C. A rate of attack expressed in nanometers per minute is deduced from the thickness measurements. With the films whose preparation has been described above, the rates of attack are on the order of 5 nm/mn.
The second method relates to a SIMS analysis of the original film as it is immediately after its deposition on a substrate where the alkaline ions become mobile when the temperature rises such as, for example, on the soda-lime-silica glass and an analysis of a similar film after it has undergone a heat treatment at 600.degree. C. for an hour under a nitrogen atmosphere. The test consists in comparing the concentration profiles of the alkaline ion--in this case, sodium--before and after the treatment to see if the diffusion has occurred or not. With the usual barrier films, especially those with a silicon oxide base, a certain diffusion is noted.